I don’t think it is that irrelevant - reattaching bits of the same soul seems to have affected density far more than strength for instance. Formerly ryne until the retcon, the wol, graham and potentially krile are all at 9/14th and not notably stronger than other denizens of the source - or more pertinently the first, ninth and thirteenth.

Taking the two souls thing literally, blessed siblings do seem to stand on a higher baseline than one souled people. Similarly zarol ja, mr snakes from vanguard and the arcadion fighters all seem to get notably stronger wearing a foreign soul. This does seem to also have some pretty bad side effects such as killing you and needing souls to use. I think zarol ja is the only individual we see fight without, with an extra and with a lot, so the main source of extrapolation about how much impact it might have.

There is the potentially similar effect from eating all the light wardens, but I don’t remember that impacting strength, just soul composition until the side effects started and ryne had to use the soul glue.
Likewise it seems that the resonant business is more of a matrix style learnt skill than stat increase, despite using a lot of people as fuel. Since zenos should have had natural echo it does raise a question of why he had the crazy eyes as well, but it might be another side effect. I don’t remember if my cup brimmeth over with ice and fire guy had the same eyes, or the hypertuned but I think they’re a bit different.